T cell tolerance has been found to occur in immature T cells in the thymus and also extrathymically in mature T cells. Our studies of mature T tolerance have revealed that these cells may undergo deletion or enter a functionally unresponsive state termed anergy. In anergy, a T-helper cell can be induced to "turn off" IL-2 production in response to typical stimulatory ligands if it is first signaled through its T-cell receptor in the absence of costimulation. Previously we showed that regulatory proteins that govern IL-2 gene expression may be poorly activatable in response to antigen in anergic T cells. One important determinant of whether T cells will be normally activated by encountering an antigen/MHC complex is whether it is presented along with a co-stimulatory stimulus. Co-stimulation is the signal provided by the interaction of surface molecules such as CD28 (on the T cell) and CD80 or CD86 (on the antigen-presenting cell). The presence of co-stimulation seems to be required for the normal T cell proliferative response to antigen. To understand the role of co-stimulation in IL-2 production, we have studied the effects of co-stimulation on various elements in the IL-2 promoter. Thus far these studies indicate that the transcriptional function of certain cis elements in the promoter can be augmented by co-stimulatory influences. Further studies are in progress to define the molecular mechanisms involved in more detail. Finally, an important aspect of the success of an immune response to an invading micro-organism appears to be the specialization of the helper T cell response towards cells that either produce IL-2 (TH1 cells) or IL-4 (TH2 cells). We have studied the regulatory elements in the promoter of the IL-4 gene and detected a transcriptional trans-activator and its binding site (CS-1) that are restricted to and crucial for expression in TH2 cells. Recently work from other labs has indicated that the promoter region containing the CS-1 site includes binding sites for NF-AT and the c-maf proto-oncogene. The c-maf proto-oncogene appears to be expressed specifically in TH2 cells. We will analyze the regulation of c-maf to understand how it governs lymphokine gene expression in the differentiation and function of TH2 CD4+ T lymphocytes. We also plan to dissect the IL-2 promoter to determine the regions that serve to positively or negatively contribute to its specialized expression in TH1 cells.